1. Field of the Invention
The present invention relates generally to a joint structure for each ball-end of an axle housing, and more particularly to a joint structure at each ball-end of an axle housing for a driving axle with a steering device.
2. Description of the Prior Art
A typical driving axle with a rigid type steering device used in an all wheel drive type vehicle has heretofore included a spherical portion equipped with a dust-tight seal so as to prevent foreign matter from entering the interior of each joint. This type of joint structure for an axle housing ball-end is disclosed, for example, in the specification of Japanese Patent Laid-Open No. 24226/1981. A follower steering shaft of the type disclosed in the above-mentioned specification will be hereinafter described with reference to FIG. 7.
Referring to FIG. 7, the follower steering shaft is essentially constituted by three components which can be separated from one another: a shank 71 coupled to a differential unit; a constant-velocity joint 72; and a steering shaft 74 for driving a boss or a sun gear (not shown) within a planetary unit. The constant-velocity joint 72 includes a spherical external coupling member 75 mounted on one end of the shank 71 and an internal coupling member 77 mounted on a corresponding end of the steering shaft 74. A bellows 73 hermetically seals the inner chamber of the constant-velocity joint 72, the bellows being detachably mounted on the external coupling member 75 of the constant-velocity joint 72, and a neck 76 of the bellows 73 being maintained in airtight contact with the steering shaft 74 for free movement along the axis thereof. The bellows 73 has an axial residual stress, and is therefore formed strongly enough to be extended to its maximum axial length. The constant-velocity joint 72 equipped with the bellows 73 can be removed without the need to release it.
In general, a conventional type arrangement has difficulties in ideally disposing the seal on the spherical portion of each of the axle housing ends since the ends of king-pins, that is, dust-tight seals are hindrances. The spherical portion at the axle housing ball-end housing the driving axle therein forms a member for supporting a load as a portion of an axle casing. For this reason, the conventional configuration of a rotary body housing an axle shaft which is worked by means of a lathe does not allow for any reduction in the diameter of the neck from the viewpoints of strength, so that a seal sliding surface necessary for satisfactory steering cannot be ensured and the maximum steering angle possible is therefore restrained up to about 30.degree.. However, since the internal structure of each joint and associated components are actually arranged to allow the maximum steering angle of about 40.degree., a demand has arisen with respect to a seal having a steering-angle function to be matched with such structure.
For the above-described reason, the prior art discloses an elliptic-seal structure in which a surface of each knuckle on which a seal is mounted is inclined with respect to the spherical portion of an associated axle housing end. This type of structure is disclosed, for example, in the specifications of Japanese Utility Model Publication Nos. 37386/1984 and 37387/1984.
First of all, a diagrammatic description will be made with respect to a front-wheel support device for a front drive vehicle described in Japanese Utility Model Publication No. 37386/1984. Either an oil seal member or an oil seal protection member is so formed as to have an axial length greater on the rear side than on the front side in the direction of movement of the vehicle when a knuckle housing is mounted. The seal of this prior art is disposed around a king-pin for pivoting the knuckle housing and a trunnion socket in an inclined manner at a predetermined angle, so that, when a vehicle is moved straight, a flat surface including the seal edge of either the oil seal member or the oil seal protection member is disposed closer to the shaft of the trunnion socket on the front side than on the rear side. In this front-wheel support device for a front drive vehicle, the seal is formed in a special shape so as to increase the steering angle. However, even if the seal is formed in such a special shape, it is not necessarily satisfactory in respect of its strength, sealing characteristics and durability. In addition, this type of seal is difficult to produce and of no practical use, thus leading to various problems.
In the second place, a diagrammatic description will be made with respect to an oil seal device for a front-wheel support of a front drive vehicle described in the specification of Japanese Utility Model Publication No. 37387/1984. Referring to an oil seal member in this prior art, a rubber-made seal body has an equal width along its entire circumference in a natural state, and a circumferential groove of a L-shaped in section is formed from a mounting flange of the seal body to an arm portion perpendicular to the mounting flange. A reinforced flange member of a metal-made cylindrical flange body has a cut surface on the cylindrical side, such cut surface being inclined with respect to a flange surface, and is adherently engaged with the circumferential groove in a state wherein this groove is under an elastic tension. The reinforced flange member is also disposed around a king-pin for pivoting the knuckle housing and the trunnion socket in an inclined manner at a predetermined angle, so that, when a vehicle is moved straight, a flat surface including the edge of the annular seal is positioned nearer the trunnion socket on the front side than on the rear side. As in the case of the above-described example, this oil seal device for the front-wheel support of a front drive vehicle needs a seal with a special shape, and thus there is a problem in that the seal provided may not necessarily be satisfactory from the viewpoint of strength, sealing characteristics and durability, nor with respect to considerations regarding the production of the seal.